1. Field of the Invention
This invention relates to a process for producing a stretched multilayer polyester bottle, and more specifically, to a process for producing a stretched multilayer polyester bottle the characteristic, of which lies in the production of a multilayer polyester preform.
2. Description of the Prior Art
It is known that biaxial stretching of a thermoplastic polyester capable of being oriented by stretching, such as polyethylene terephthalate, improves its appearance such as transparency, its mechanical properties such as rigidity and its dimensional stability. Biaxial stretching has long been utilized for molding of containers. A container having a biaxially oriented wall is produced by extrusion-molding or injection-molding the aforesaid thermoplastic polyester into a bottomless or bottomed cylindrical preform, and simultaneously or successively carrying out an operation of stretching the preform axially and an operation of expanding the preform circumferentially by blowing a fluid.
The biaxially stretched container has excellent transparency, rigidity, impact strength and dimensional stability, but does not prove satisfactory in its gas-barrier property. For example, a biaxially stretchable polyester such as polyethylene terephthalate, even after stretching, has a far higher oxygen permeability than a resin having high gas-barrier property such as a saponification product of an ethylene-vinyl acetate copolymer or a vinylidene chloride resin. On the other hand, the resin having high gas-barrier property is generally difficult to stretch biaxially.
It has already been known that to improve the gas-barrier property, particularly the oxygen-barrier property, of a biaxially stretched container, the orientable thermoplastic polyester is used in combination with an oxygen-barrier resin to provide a biaxially stretched multilayer container. For example, U.S. Pat. No. 3,733,309 discloses that in the production of a stretched blow-molded container from a preform of a polyester, an improvement in its gas-barrier property is expected by making the preform in a multilayer structure composed of the polyester and an ethylene-vinyl alcohol copolymer. The U.S. Patent, however, gives no specific disclosure about how this multilayer preform is produced.
For the production of a multilayer preform, multi-stage injection molding of a plurality of resins has previously been known (see, for example, Japanese Laid-Open Patent Publication No. 111,236/1980). The production of a multilayer preform of the aforesaid polyester and ethylene-vinyl alcohol copolymer by this multi-stage injection molded method gives rise to a problem in that the thickness of the ethylene-vinyl alcohol copolymer layer is difficult to adjust to not more than 1.0 mm in view of the balance between the melting--flowing and the cooling--solidification of the copolymer. The ethylene-vinyl alcohol copolymer with a thickness above 1.0 mm is very difficult to stretch under the polyester stretching conditions and economically, too, it is desired to reduce the thickness of the ethylene-vinyl alcohol copolymer layer.
Japanese Laid-Open Patent Publication No. 75826/1982 discloses a method which comprises coating a preform of an orientable thermoplastic resin such as a polyester with a solution or emulsion of a resin other than the ethylene-vinyl alcohol copolymer, for example a resin, such as a vinylidene chloride copolymer, which is impossible or difficult to heat-mold and has adhesiveness and gas-barrier property, providing a layer of the orientable thermoplastic resin further on the coated surface by injection or extrusion to form a multilayer parison, and subjecting the multilayer parison to biaxially stretching blow molding. However, by a method of forming a multilayer preform by injection molding the polyester in two stages with a step of coating the gas-barrier resin interposed therebetween, the gas-barrier resin layer at the bottom portion of the preform which is near the gate of the injection mold moves toward the neck portion by the influence of the pressure of injecting the polyester, and consequently becomes very thin. Furthermore, in the step of drying the gas-barrier resin solution or emulsion, the polyester layer may be whitened by crystallization and decrease in stretchability. Or the polyester layer will lose transparency.
The present inventor has found that these problems can all be solved by coating a solution or emulsion of an ethylene-vinyl alcohol copolymer on a primary preform of a polyester, drying the preform at a temperature at which no whitening of the polyester owing to its crystallization occurs, and thereafter injecting the polyester onto the coated film.